Characteristic Modes in CST

I want to simulate a dual band antenna using CST MWS. I'm using the multilayer solver to find the different characteristic modes at resonances. When Multilayer solver parameters window appears, it asks to indicate a frequency for mode sorting. My question is which frequency for sorting i should use since i've a dual band antenna? I've thought of using two identical models, each one is using a different frequency for sorting. Is this correct?